1. Field of the Invention
The present invention relates to a system for broadcasting a scrabled television signal.
2. Description of the Prior Art:
Recently, "pay-TV" systems have become popular in the U.S.A. In the pay-TV systems, viewers who have contracts with the broadcasting station can correctly receive a program, while viewers who do not have contracts with the broadcasting station cannot. In order to prevent correct reception by nonsubscribers, methods for processing video and audio signals in some manner are used. According to such a method, when a program is received by a television set of a nonsubscriber, he cannot obtain correct picture and voice.
Thus, these methods are called scrambling methods or scrambled broadcast. For this purpose, various scrambling methods for processing video and audio signals have been proposed. Then, in the pay-TV system using such a scrambling method, a viewer who has a contract with the station is provided a decoder for decoding (or descrambling) the scrambled signal to obtain the original signal.
One scrambling method is known in which a television signal is inverted at a predetermined turn-up level with respect to white and black levels. For example, a video signal S.sub.v as shown in FIG. 1A is inverted at a turn-up level V.sub.m set between white and black levels to obtain a signal S.sub.vs as shown in FIG. 1B. In this case, if horizontal and vertical synchronizing signals of the television signal are inverted, the original signal cannot be reproduced by a television set of a subscriber. Therefore, only the video period is inverted.
When such a scrambling method utilizing level inversion is adopted in actual broadcasting, a scrambling pattern is formed by combining level-inverted and non-inverted fields. The obtained scrambling pattern is repeated to transmit the scrambled television signal.
An identification signal which indicates that the broadcast program is scrambled and a key code signal for allowing descrambling of the program received by a television set of a subscriber are inserted at predetermined positions of a vertical blanking period. In one scrambling method, for example an identification signal of a predetermined level is inserted in the sixteenth scanning period of the vertical blanking period and a key code signal is inserted in the seventeenth and eighteenth horizontal scanning periods (to be referred to as 17H and 18H hereinafter).
When a television signal scrambled by this method is received, the inverted field period is detected by the key code signal, and the video signal in this period is reinverted. According to this method, when a scrambled signal is subject to a distorting influence by a transmission system, descrambling at a receiver side may not result in reproduction of an original waveform.
For example, when a polarity-inverted signal is reinverted by a decoder, a level difference may be generated or the waveform may be distorted. When a descrambled signal is supplied to a TV receiver, flicker occurs at the polarity-inverted portion of the picture. Such flicker is particularly noticeable in the case of a still image. When a scrambled program broadcast from a station is recorded by a VTR, flicker tends to become more noticeable since a VTR is nonlinear in a DC manner as well as in an AC manner.
The present applicant has previously proposed a scrambling broadcast system free from this problem in Japanese Patent Application No. 57-13669 and the like. This system will be described briefly below.
Referring to FIG. 2A, a predetermined period T (e.g., 60 seconds) of a television signal of a specific program to be scrambled is preset by an identification signal ID. A scene change of such a program is detected so as to obtain a scene change signal SC. Then, a key signal pattern P.sub.1 as shown in FIG. 2B is prepared. The leading and trailing edges of the pattern P.sub.1 coincide with the field period.
In accordance with the signals ID and SC and the pattern P.sub.1 as described above, one of scrambling patterns P.sub.2, P.sub.3 and P.sub.4 (FIGS. 2C, 2D and 2E) is generated which determines the timing of the polarity inversion. Scrambling is performed by inverting the polarity of a video signal period of a television signal of a specific program in accordance with a high level portion of the pattern P.sub.2, P.sub.3 or P.sub.4, for example. The key signal pattern P.sub.1 is repeated at the period T.
The pattern P.sub.2 is obtained by ANDing the signal SC and the pattern P.sub.1 and inverting the level by the AND oroduct. The oattern P.sub.3 rises with the signal ID or SC and falls at the trailing edge of the pattern P.sub.1. The pattern P.sub.4 rises with the AND product of the signal ID or SC and the pattern P.sub.1 and falls at the trailing edge of the signal SC. Various other scrambling patterns for determining the timing of the polarity inversion may also be formed in accordance with the signals ID and SC and the pattern P.sub.1.
According to the method described above, the polarity inversion is always performed together with a scene change. Scene flickering can thus be deemphasized.
However, since the polarity inversion of the scrambled signal is limited to the scene change timing, it is performed about once for every few seconds. When this scrambled signal is received by the conventional receiver, the normal scene is displayed for a few seconds to several tens of seconds when the polarity is normal. In addition, even if the polarity of the signal is inverted, the motion of the people and characters can be easily understood when the inverted period is long. Therefore, the scrambling effect becomes low, thereby impairing secrecy as the main feature of scrambled broadcast.